Enzymatic hydrolysis of proteins by means of proteases is a well-established method of preparing protein hydrolysates which have retained the nutritional value of the original proteins and which may therefore advantageously be used in the diet of certain patients who are unable to ingest or digest sufficient amounts of full-length proteins present in ordinary food, or which may be used to improve the nutritional value of milk substitutes for infants. Protein hydrolysates may furthermore be prepared from sources not traditionally utilized for human nutrition and may, as such, either be used as food products in themselves or as additives to other food products.
The proteases which have hitherto been used for the preparation of protein hydrolysates of this type are proteases with a broad specificity, e.g. Bacillus licheniformis alkaline protease. A major problem encountered when using proteases with a broad specificity is an often pronounced bitter taste of the protein hydrolysates produced. The bitter taste is the result of cleavage of the proteins at amino acids with hydrophobic side chains, resulting in the formation of peptides with exposed hydrophobic amino acid residues. In full-length proteins and longer peptides, the hydrophobic side chains are inaccessible due to the tertiary structure of the protein molecule in which the side chains are embedded within the folded protein. When smaller peptides are formed by proteolytic cleavage of the protein molecule, the hydrophobic side chains will become exposed and hence accessible to adjacent hydrophobic and hydrophilic receptors on the taste buds. This phenomenon has been determined to give rise to a bitter taste (cf. H. Wieser and H. -D. Belitz, Z. Lebensm. Unter.-Forsch. 159, 1975, pp. 65-72; and H. Wieser and H. -D. Belitz, Z. Lebensm. Unter.-Forsch. 160, 1976, pp. 383-392).
It has been suggested to overcome the problem of bitterness of the protein hydrolysate by limiting the degree of hydrolysis of the starting proteins, i.e. by limiting the number of peptide bonds cleaved by the protease, e.g. by monitoring the degree of hydrolysis and terminating the proteolytic reaction when a suitable degree of hydrolysis has been obtained (cf. for instance J. Adler-Nissen, Enzymic Hydrolysis of Food Proteins, Applied Science Publishers, London 1986). It has been found that such hydrolysates exhibit a reduced bitterness, at least in conjunction with other constituents of the food product in which they are included.
Another way of controlling the degree of hydrolysis is to employ specific proteases which only cleave the protein molecule at certain amino acid residues. This has been suggested by J. -M. Chobert et al., J. Agric. Food Chem. 36, 1988, pp. 220-224, reporting the use of the Staph. aureus V8 protease which hydrolyses proteins specifically at glutamic and aspartic acid residues.